Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude

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Abstract

Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forêts, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress SH. A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forêts (where the magnitude of SH is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of SH from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of SH based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.

Introduction

Drilling-induced compressive and tensile failure of wellbores is a well known phenomenon in hydrocarbon industry. Compressive failure leads to oriented enlargements of the borehole cross-section known as wellbore breakouts1, 2. Unintensional hydraulic fracturing induced by too high mud pressures is known to cause loss of circulation in wells3, 4. The orientation of wellbore failures can be determined with geophysical logging tools such as the Borehole Televiewer (BHTV)[5]or Formation MicroScanner/Formation MicroImager1 (FMS/FMI) tools[6].

In this paper we focus on a type of wellbore failure, drilling-induced tensile wall-fractures, revealed by detailed FMS/FMI logging in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forêts, France. A phenomenological description of this failure type and an analysis for the purpose of the determination of the state of stress is provided. These fractures are observed in pairs, mostly parallel to the nearly vertical wellbore axes and on diametrically opposed sides of the borehole walls. Fig. 1 shows two examples of such fractures recorded with the Formation MicroScanner (FMS) and the Formation MicroImager (FMI) in the KTB pilot hole (Fig. 1a) and in the GPK1 borehole at Soultz-sous-Forêts (Fig. 1b), respectively. The fractures are seen as near vertical stripes of reddish colors, indicating high electrical conductivity presumably because drilling mud invaded the fractures. In this paper we will thoroughly describe the phenomenology of the fractures and try to explain their initiation, their orientation, occurrence with depth and trace at the wellbore wall. We will also give an explanation for the initiation of the fractures analyzing the various stresses acting on the borehole wall during drilling operation and finally show how the occurrence of these fractures can be used for the estimation of the magnitude of the greatest horizontal principal stress SH under the precondition that the least horizontal principal stress Sh is known from another method like hydraulic fracturing (e.g. Refs.7, 8).

The KTB project[9]was conducted in two phases. Starting in September 1987, a 4000 m deep pilot hole was drilled close to the village of Windischeschenbach in Bavaria, Germany. The KTB main hole, started in October 1990, reached a final depth of 9101 m in October 1994. Both wells were drilled through steeply dipping gneisses and amphibolites, the main hole encountering several major zones of faulting, which were detected seismically10, 11and by borehole measurements and also in the cores and cuttings12, 13, 14. An extensive logging program provided a complete data set of physical, lithological, chemical and technical parameters to a depth of ∼8700 m. The KTB pilot hole provided also an almost complete set of cores to a depth of 4 km. Due to the use of a motor steering system to a depth of about 7600 m in the KTB main hole the deviation was kept below 1° to this depth, but increased to about 20° at great depth.

The borehole at Soultz-sous-Forêts[15]is situated in the western part of the Rhine Graben, a rift system extending about 300 km from Basel to Mainz and belonging to the Mid-European Rift System. The location is close to the center of a pronounced geothermal anomaly situated in the Rhine Graben. The geothermal gradient in the Soultz-sous-Forêts area is about 3 times higher than the worldwide average gradient of 30°C/km. The Granitic Soultz Horst is covered by about 1400 m of sediment. Faults trending approximately N–S characterize the tectonic structures of the area. The well was drilled into the basement consisting of relatively homogenous porphyritic granite to a depth of 3590 m.

Section snippets

Detection and phenomenology of drilling-induced tensile fractures

Formation MicroScanner (FMS), Formation MicroImager (FMI) and in the case of the KTB main hole also High Temperature Formation MicroScanner (HFMS) were used to gain a picture of natural faults, fractures and foliation of the rock crosscut by the borehole. All three instruments are based on the same physical principal[6]. Small, circular electrodes (diameter ∼4 mm) located on four extendible arms which are hydraulically pressed at the wellbore wall emit an electrical current into the rock. The

Fracture traces inclined to the borehole axis

As mentioned above tensile fractures which are inclined to the wellbore axes were found in all three boreholes. Theoretically, this inclination can be caused by the well axis not being parallel to a principal stress[26]. This could mean either that the borehole is vertical like the KTB main well and the vertical stress is not a principal stress, or the borehole is deviated from the vertical direction if Sv is a principal stress. The analytical relations describing the stress field in an elastic

Initiation of vertical drilling-induced tensile fractures

To discuss the initiation and development of the drilling-induced tensile fractures we assume in the following that the vertical stress is a principal stress and that the borehole axis is vertical. Cases where these assumptions do not hold are discussed separately below. We further assume that the stress field around the wellbore can be described by the analytical solution given by[36]for the stress field around a circular opening in an elastic, isotropic infinite plate which is loaded by the

Confirmation of proposed model for fracture initiation and estimation of Sh magnitude from analysis of drilling-induced tensile fractures

Magnitudes of the horizontal principal stresses Sh and SH determined by hydraulic fracturing in the GPK1 well and in the KTB pilot hole allow to demonstrate that the initiation of drilling-induced fractures can be described by the model proposed above. This model also offers the opportunity to estimate the magnitude of SH if Sh is known from any other stress measurement method. Using Eq. (2)it can be argued that fractures initiate first if the tangential stress at the borehole wall σθθ is equal

Stress estimation using analytically calculated stress intensity factors

The analysis presented above is based on the description of tensile failure at borehole walls following Hubbert and Willis[7]. The initiation and growth of tensile fractures also can be described by the analysis of the stress intensity factor KI which describes the stress concentration at the fracture tip which is responsible for either growth or stabilization of the fracture. If the stress intensity factor for certain loading conditions overcomes a critical value which is specific for the

Conclusions

The theory presented for the initiation and development of the drilling-induced tensile wall-fractures is based on stress analysis for a linear elastic material using a strength of material approach for the initiation of the fractures. The initiation of the fractures can be explained taking into consideration the tectonic stress state, the downhole pressure acting on the borehole wall and the thermal stresses in the borehole wall induced by circulation of relatively cold drilling mud. This is

Acknowledgements

The authors like to thank J. Kück and M. Sowa for their continuous help in accessing the borehole measurements from the KTB boreholes and K. Huber for insightful discussions. The authors also wish to thank R. Nagel who analysed the FMI data from the GPK1 borehole for the occurrence of drilling-induced tensile fractures. Data from the KTB boreholes were kindly provided by NLfB-KTB and data from Soultz-sous-Forets by SOCOMINE. This work was financially supported by the Deutsche

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